The present invention concerns a vehicle braking system, and particularly an air braking system of the kind used on commercial vehicles.
Light vehicles such as cars and vans use dual circuit hydraulic braking systems, often with servo assistance. Heavier vehicles such as trucks and buses often use air braking systems in which air under pressure is the working medium.
A typical air braking system will include an air compressor, an air reservoir, a distribution valve, two service brake circuits and a handbrake circuit. Other components such as an air dryer and oil filter may be provided to ensure that the air supplied is at a desired quality. In addition to the usual three braking circuits air may also be supplied to a low pressure auxiliaries circuit where it is used for example for gear shift assistance or drivers seat adjustment. A high pressure auxiliary circuit may be provided, for example for vehicle air suspension control.
The distribution of air under pressure between the respective circuits is highly complex. As a priority air must be supplied to the brake circuits, but for example it is undesirable for a handbrake circuit to permit release of the handbrake whilst air pressure in the service brake circuits is low. The braking system must have carefully designed distribution arrangements to ensure that an air leak in one circuit does not result in loss of air pressure in all circuits; these arrangements may be very complex in order to give effective braking under all likely failure conditions. The air compressor must be large enough to meet all reasonable needs, but not to be so large that power consumption is unnecessarily high. The components of the braking system are preferably designed to meet the alternative legislative requirements which may be in force in different countries. For example legislation may limit the maximum usable pressure in some circuits but not in others. Finally the cost of the braking system must be maintained at a reasonable level for competitive reasons.
A conventional air braking system usually includes a rather complicated valve block which houses numerous valves, restrictors and other fluid components, the purpose of which is to direct air from the compressor to the air consumer circuits in the desired manner. Each valve block is designed for a particular vehicle installation and, once manufactured, is not susceptible of modification or adaptation. The danger of contamination is such that tampering or repair is discouraged. The manufacturing tolerances of a valve block must be very precisely controlled in order to ensure that the braking system operates correctly. The requirement for close tolerances leads to a consequential increase in manufacturing cost, and the potential for a high scrap cost if a mistake is made in the manufacturing process.
In order to reduce costs it would be desirable to reduce the number of different valve blocks. Furthermore it would be desirable to be able to adapt a valve block to generally different circumstances of use without requiring a re-design of the fluid components therein. It would also be useful to be able to tune a valve block to the precise circumstances of a particular vehicle application.